Several types of chemical analytical equipment utilize a pump to propel the analyte through the equipment. For example, a pump is utilized to pump the analyte through a high performance liquid chromatography (HPLC) column and any attached equipment, such as a guard column. Pumps used in chemical analytical equipment often utilize a reciprocating piston that is housed external to the pump housing within a pump head assembly. With the passage of time, the piston may malfunction or, more commonly, the seals within and around the pump head become worn. Consequently, it is desirable that the pump head assembly be designed so that it is easily and quickly removable in order to facilitate repair or replacement of worn or broken parts.
Additionally, it is desirable that the pump head assembly be designed to avoid cocking when the pump head is tightened against the pump housing. Cocking occurs when force is unevenly applied to the pump head as it is tightened against the pump housing, thereby causing an uneven seal between the pump head and the pump housing and a potential misalignment between the piston and piston cavity including the seal guide and fluid chamber. For example, most manufacturers mount the pump head to the pump housing by means of two bolts that penetrate the pump head body and which are threadably inserted into the pump body. The bolts are individually tightened, and if one bolt is tightened more than the other then the uneven force applied to the pump head may cause cocking.
One conventional approach to prevent cocking in a pump head assembly is to externally mount a bracket over the pump head. The bracket does not directly contact the pump head, but includes a centrally-located set screw that is tightened against the pump head to ensure a seal between the pump head and the housing of the pump. This design avoids cocking, by centrally applying a compressive force so that it is evenly distributed across the pump head surface that contacts the pump housing. Removal of the pump head can also be completed more rapidly. A drawback to this design, however, is the requirement for two brackets; the bracket mounted over the pump head, and a support bracket, attached to the pump housing, which engages the bracket mounted over the pump head. Thus, this type of pump head securement cannot be readily applied to convert existing pumps. To date, this design has been utilized only in certain HPLC systems incorporating a pump head which is secured to the piston within, so that the external bracket must first be removed, to liberate the pump and pump head assembly, before the pump head can be separated from the pump.
Another conventional solution to the problem of designing a quick release pump head assembly is to mount a cover plate over the pump head by means of two bolts which penetrate the cover plate and the pump head. The bolts are threaded into passages defined in the pump head housing. Within the pump head the bolts pass through hollow sleeves to prevent binding of the bolts to the inner surfaces of the pump head passages. The bolts support the pump head, but do not serve to directly tighten the cover plate against the pump head. The only point of contact between the cover plate and the pump head is an adjustable, centrally-located screw that penetrates the body of the cover plate. Tightening of the screw applies a force against the pump head which seals the pump head against the pump housing. Again, this design avoids cocking by centrally applying a compressive force that is evenly distributed across the pump head surface that contacts the pump body.
A drawback to this design, however, is that to remove the pump head it is necessary to first loosen the set screw and then use a wrench to unscrew and remove the two bolts that attach the cover plate to the pump head and the pump head to the pump housing. Thus this design does not provide for quick removal of the pump head. Another serious drawback to this design is that once the bolts and their hollow sleeves have been removed, the weight of the pump head is borne solely by the delicate piston located within the pump head. The piston may be broken or bent by the weight of the pump head, in which case the piston seals may also be damaged.
Further, when the pump head of conventional quick release systems is being mounted onto the pump, the pump head must be manually guided and inserted straight along the axis of the piston in order to avoid damaging the piston or the piston seals.